Vibration absorbers are commonly used in diverse industries to reduce the level of vibration in a variety of structures. For instance, a passive vibration absorber is a simple and effective device which is widely used to suppress undesirable machine vibrations excited by harmonic forces.
One classic form of passive vibration absorber has a mass-spring-damper configuration, which consists of a small absorber mass connected to a primary system of a vibrating structure by a spring and/or a damper. Typically, the vibration absorber is designed to have spring and mass parameters that give it the same resonance frequency as the vibrating structure. As a result, the inertia of the vibration absorber can reduce the net force on the primary system, hence attenuating the vibrations at this frequency. The vibration absorber effectively adds a large impedance to the primary system.
However, such passive vibration absorbers are generally only effective over a narrow frequency range. When the excitation frequency varies widely, the vibration attenuation effect of the vibration absorber would decrease rapidly due to frequency mistuning. Therefore, passive vibration absorbers are not effective or become inapplicable in many applications with varying excitation frequencies.
One conventional approach to solving this problem was the development of adaptive or active vibration absorbers to expand the effective operational bandwidth. Active vibration absorbers can improve vibration attenuation by adjusting their own vibrating frequencies to track the changing excitation frequency. In an active vibration absorber, an active force component is controlled by an active control system and is positioned between the absorber mass and the primary system of a vibrating structure. This device provides adaptability and improved performance in some areas of vibration control. Active control has been used effectively with lightweight actuators such as piezoelectric materials. Heavier and more powerful electromagnetic actuators have proven to be effective on heavier structures and systems.
On the other hand, adaptive vibration absorbers use the technique of altering their design parameters automatically to control their resonance frequency to match that of the primary system of a vibrating structure. In one example, the absorber resonance frequency is tuned by changing a spring stiffness of a dampener using a stepper motor. Along with actuator design, a lot of research in active control comprises the investigation of different control strategies used for actuation: neural networks, delayed resonator, modal feedback controller, and disturbance cancellation. For example, the delayed resonator utilizes feedback regarding positions of the absorber with a controlled time delay to push the absorber to resonate at the same frequency as the excitation.
For semiconductor processing machines such as wire bonders, machine vibrations demonstrate characteristics of varying excitation frequencies (or motion cycle times) within a relatively large range, and the excitation frequencies tend to change very frequently.
For the adaptive vibration absorbers that use motors to adjust spring stiffness with feedback control algorithms, slow adjustment time and poor reliability under extended operations make them impractical for use in semiconductor processing machines.
Conventional active vibration absorbers use sensors to monitor excitation frequency and to monitor the vibration of the absorbers themselves and/or their primary system. After processing the monitored information, a feedback control signal is generated to produce a force which acts on the vibration absorber. As a result, the vibration absorber vibrates and damps the vibration of primary system at the monitored excitation frequency. However, this approach has several drawbacks if used in semiconductor processing machines. First, it requires a relatively long time for data processing (usually involving Fast Fourier Transform or “FFT” processing) as compared to motion cycle time. Due to the nature of feedback control, the whole system has a long transient time and poor transient performance for fast varying excitation frequencies. Furthermore, it generally requires several sensors to collect vibration signals, increasing the costs of adopting this approach. If the absorption of more than one frequency is required, typically more than one vibration absorber or a vibration absorber with multiple degrees of freedom is required.
Therefore, in order for a vibration absorber to be useful in semiconductor processing machines, it must be tunable to a range of excitation frequencies and this tuning must be fast enough to respond to varying excitation frequencies during operation. It would be better to use motion commands of the primary system itself instead of signals from separate sensors to generate control signals. It would also be preferable that the use of one vibration absorber be effective to reduce vibration in more than one frequency and the vibration absorber is capable of working along more than one axis (for example, in both x and y directions).